The present invention relates to an image recording method and an image recording apparatus in which a clearance or gap between a photosensitive medium and a developing roller of a developing unit can selectively be determined in a convenient manner.
There are proposed a variety of color image recording methods and color image recording apparatuses which are based on the electrophotographic principle. Among them, such type of color image recording apparatus in which a color image is formed on a surface of a photosensitive medium constituted by a rotating drum, belt or the like and transferred to a recording paper or sheet is excellent over the others in view of high speed operation and continuous paper or sheet recording specifications.
In the recording apparatus in which a color image is once formed on a surface of a photosensitive medium and then transferred to a recording sheet and in which a photosensitive medium of a drum-like structure is employed, a plurality of electrostatic latent image forming units and developing units for developing the latent images in different colors, respectively, are disposed along the rotating direction of the drum-like photosensitive medium in opposition to the outer peripheral surface thereof, wherein a toner image of a first color is formed on an image forming region of the photosensitive drum, which is then followed by formation of a toner image in a second color. In this way, the image developing process is repeated a number of times.
Thus, in the case of the recording apparatus of the type described above, the development of a color toner of a second latent image is performed on the surface of the drum-like photosensitive medium on which the toner image of the first color has been formed. Consequently, when the developer (i.e. developer or agent) is brought into contact completely with the surface of the photosensitive medium, the toner image of the first color will possibly be scraped off by the developing device upon forming the second color image, as a result of which image density of the first color is deteriorated, presenting thus a problem that the color mixing is brought about upon image development of the second color because of mixing of the toner of the first color with that of the second color. Same problem may similarly arise in association with the developing devices or units for the third and other subsequent colors.
As approaches proposed heretofore for solving the above problem, there may be mentioned techniques disclosed in Japanese Examined Publication No. JP-B-63-43748 and Japanese Unexamined Publication No. JP-A-56-144452. According to the technique described in the first mentioned publication, it is taught in conjunction with the contact of a magnetic brush formed on a developing roller of a developing unit with the photosensitive medium that a sliding contact force of the magnetic brush of a second developing unit which is exerted to the photosensitive medium surface is selected smaller than that of the magnetic brush of the first developing unit, so that upon development of the image in a second color, the toner image of the first color is protected from being scraped off by the magnetic brush of the second developing unit.
On the other hand, in the case of the recording apparatus described in the second mentioned publication, a developer or toner of the second developing unit is held contactless relative to the photosensitive medium, wherein an agitator device is provided for imparting agitation to the developer for thereby allowing the developer to form a magnetic brush on an outer peripheral surface of a developing roll for developing a toner image on the drum-like photosensitive medium.
In either of the recording apparatuses disclosed in both publications mentioned above, a clearance or gap is provided between any one of the developing rollers and the photosensitive medium, wherein the magnetic brushes are formed on an outer peripheral surfaces of the developing rollers, respectively. However, in order to realize a color image of satisfactory quality, there arises necessity for determining definitely a relation between the gap and a height of the magnetic brush. Additionally, it is required to secure the density of the toner image of a first color as well as that of the second color toner image. In other words, an attempt to secure the toner image density of the first color is usually accompanied with lowering and/or nonuniformity in the toner image density of the second color. On the other hand, attempt for securing the toner image density of the second color tends to result in appearance of a fog which lowers the toner image density of the first color. Besides, consideration will have to be paid to the degree of contact between the developing roller and the magnetic brush in the developing area as well as the image quality.